Stripline flex circuit

ABSTRACT

The invention removes copper from the concave side of a flex circuit around a bendable region and replaces it with a conductive epoxy to allow it to be formed to tighter bend radii than would otherwise be possible. After the flex circuit is shaped in a tight radius and attached to a mechanical structure, the conductive epoxy is cured to act as functional replacement of the removed copper.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made under a contract with the government of UnitedStates of America under contract No. 00014-02-C-0068 and with governmentsupport. The government has certain rights in this invention.

BACKGROUND

1. Field of the Invention

This invention is directed to flexible printed circuit boards, andparticularly, to a device and method for a flexible printed circuitboard incorporating stripline or microstrip transmission lines that passthrough a small radius bend.

2. Related Art

Flexible printed circuit boards or “flex” circuits are used in a widevariety of applications, where an electrical circuit must bend aroundcorners or be flexed during operation. Typically, flex circuits arethin, light weight, flexible, and exhibit high routability. Generally, aflex circuit may be used as an interconnecting medium in a phased arrayarchitecture. In some cases, particularly when microwave signals arepresent, design considerations mandate that the flex circuit is astripline construction of certain minimum thickness; which typicallyconsists of a central trace sandwiched between two ground planes, whichare spaced a certain distance apart. Two interposing low-loss dielectricmaterial layers are used as insulators. Alternately, the flex circuitmay feature a microstrip construction; which typically includes a traceand a single ground plane, spaced a specific distance apart, with alow-loss dielectric material as an insulating interposer.

Generally, there is a minimum bend radius to which flex circuits may beformed without damaging the flex circuit. The minimum bend radius is afunction of several aspects of the flex circuit geometry and thematerials used, but the distance between the outermost metal layers ofthe flex circuit is a key parameter limiting the minimum bend radius.

Many flex circuits have only one metal layer, or the distance betweenthe outermost metal layers is minimized, so that the minimum allowablebend radius may also be minimized. Unfortunately, in some cases thedistance between the outermost metal layers cannot be decreased below aparticular value due to electrical design considerations ormanufacturing limitations. This is often the case with flex circuitsthat incorporate a stripline or microstrip construction.

When a flex circuit having two or more metal layers is formed to a bendradius that is less than allowable minimum, the external copper layersof the circuit tend to crack or buckle. Internal delamination has alsobeen observed. In some cases concerning a flex circuit with a striplineconstruction, one or more central traces have broken, resulting in opencircuits. This results in low manufacturing yields, and raises seriouslong-term reliability concerns. Typically, the copper ground plane onthe convex side of the flex circuit cracks while the copper ground planeon the concave side buckles. When no cracking occurs, it is oftenbecause internal delamination has provided strain relief, sufficient toprevent cracking, but such delamination leads to additional reliabilityproblems.

What is needed is a structure and method that allow bending of the flexcircuit around a small radius while preserving both the mechanical andelectrical integrity of the design.

SUMMARY

The invention provides a device and method for forming a flexibleprinted circuit board to a smaller bend radius than would otherwise bepossible without damaging the circuit. This is done by removing copperfrom the concave side of the flex circuit in the bend region andreplacing it with conductive epoxy in an uncured or semi-cured state.After the flex circuit is formed into a small radius bend, theconductive epoxy is cured to act as a functional replacement of theremoved copper.

In one aspect of the invention, a method is provided for forming aconformable circuit element. The method includes depositing a conductivelayer on a first side of a flex circuit; etching the conductive layer toform an etched region; depositing a conductive epoxy on the etchedregion; bending the flex circuit along a bending axis to form a concavesurface on the first side; and curing the conductive epoxy.

In another aspect of the present invention, a flexible circuit isprovided including at least an outside metal layer and an inside metallayer. A first dielectric layer is interposed between the outside metallayer and the inside metal layer. The inside metal layer includes anetched-out area. A layer of conductive epoxy is deposited on the insidemetal layer having the etched-out area.

This brief summary has been provided so that the nature of the inventionmay be understood quickly. A more complete understanding of theinvention may be obtained by reference to the following detaileddescription of embodiments thereof in connection with the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features and other features of the present invention willnow be described with reference to the drawings. In the drawings, thesame components have the same reference numerals. The illustratedembodiment is intended to illustrate, but not to limit the invention.The drawings include the following Figures:

FIG. 1 illustrates a three dimensional packaging architecture for aPhased Array Antenna Element, in which a flexible printed circuit istypically used;

FIGS. 2A and 2B shows a stackup of a multi-layer flex, in accordancewith an embodiment of the present invention;

FIG. 3 shows a bending geometry of the multi-layer flex of FIGS. 2A and2B in accordance with an embodiment of the present invention;

FIG. 4 shows locations of the trouble spots associated with prior artsolution;

FIG. 5A shows a side view of a bent flex circuit in accordance with anembodiment of the present invention;

FIG. 5B shows a front view of the bent flex circuit (having a striplineconfiguration) of FIG. 5A in accordance with an embodiment of thepresent invention;

FIG. 6B shows a front view of the bent flex circuit (having a microstripconfiguration) of FIG. 6A in accordance with an embodiment of thepresent invention;

FIG. 7 shows a plot of an insertion loss captured on a network analyzerin accordance with an embodiment of the present invention;

FIG. 8 shows a plot of a return loss captured on a network analyzer inaccordance with an embodiment of the present invention; and

FIG. 9 is a flowchart showing a method of producing a flex circuit inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

As shown in FIG. 1, a multi-chip, three-dimensional packagingarchitecture 100 (hereinafter “module 100”), includes a pair of chipcarries 110 and 110A mechanically attached to a mandrel 104.Electrically and mechanically coupled to the chip carries 110 and 110Aare bridge PWBs 112 and 112A, respectively. A bent flex circuit 102provides electrical connection to bridge PWBs 112 and 112A. Guard shims108 and 100A are attached between chip carriers 110 and 110A and bridgePWBs 112 and 112A, respectively, and lids 106 and 106A are used to coverthe exposed surface of chip carriers 110 and 110A. An apertureintegrated wiring board (AIPWB) 114 is attached to mandrel 104 andelectrically connected to chip carriers 110 and 110A.

In one embodiment, bent flex circuit 102 may be delivered in an unbentform, having the stack-up shown in FIG. 2. Flex circuit 102 includes aconductive paste epoxy 202 (hereinafter “epoxy 202”) used to form a baseof flex circuit 102, and used to contact mandrel 104 (FIG. 1). Epoxy 202is cured into a semi-solid state referred to as “b-stage”.

Stacked on epoxy 202 is first metal layer 204, first dielectric layer206, prepreg layer 208, second metal layer 210, second dielectric layer212, and third metal layer 214. The metal layers may include anysuitable metal material, such as copper.

In one embodiment, flex circuit 102 may be formed to a bending profile,as shown in FIG. 3, where r0 302 and r1 304 are the internal bend radii.In this embodiment, flex circuit 102 is aligned and clamped to mandrel104. Mandrel 104 with flex circuit 102 are then inserted into a formingtool (not shown). In one embodiment, the forming tool has spring-loadedrollers that gently bend flex circuit 102 conforming it to the shape ofmandrel 104. Additional clamps are placed on the outside of the bentflex circuit 102 and the assembly is placed in an oven to finish curingb-stage epoxy 202. Bent flex circuit 102 is attached to mandrel 104,which provides the mechanical structure for module 100 (FIG. 1).

In one embodiment, the internal bend radius of flex circuit 102 may bebetween about 0.040 and 0.060 inches to accommodate half-lambda (λ/2)element spacing, where λ is the wavelength of the antenna frequency. Forexample, the λ/2 element spacing dictates a module spacing that in turndictates a bend radius of about 0.050 inches at 30 GHz. The bend radiusis scaleable with the inverse of antenna frequency. However, in practicethe larger, lower frequency antennas have additional requirements formulti-beam capability that require more space for interconnects. As aresult, the internal bend radius required to meet operational objectiveshas remained relatively constant over a frequency range of 8 GHz to 30GHz. In the current example, 0.056 inches is satisfactory.

The thickness of flex circuit 102 may be determined by the spacingrequired between the outer ground planes; which is in turn determined bythe dielectric constant of the substrate, the width of the internaltransmission lines, and the desired characteristic impedance of thetransmission lines. In one embodiment, practical limits on theseparameters dictate that flex circuit 102 be about 0.013 inches thick,excluding the thickness of the exterior ground planes. The thickness ofthe exterior ground planes is on the order of 0.001 inches, thus most ofthe thickness of the flex circuit is due to the spacing between theexterior ground planes.

Historically, problems occur when a flex of the thickness noted above isformed to the previously described internal bend radius. The problemsinclude cracks on the surface, after the flex circuit is formed aroundmandrel 104. In addition, metal can pull away from the dielectriccausing delamination. In addition, buckling of the backside metal candevelop. FIG. 4 shows the typical location where these problem areasoccur.

The Institute for Interconnecting and Packaging Electronic Circuitsmaintains IPC-2223 as the design standard for flex circuit construction.Section 5.2.3.4.2 and FIG. 5-7 of the Nov. 1998 edition set limits onthe strain the copper can sustain in different situations. This standardalso provides means of estimating the minimum bend radius thatcorresponds to the limiting strain. Table 1 from IPC-2223 listsapplicable strain limits for rolled annealed copper and electrodepositedcopper. The value for rolled annealed copper is applicable only ifrolled annealed copper foil is used, and if no copper is electroplatedover the top of the foil. In one embodiment, flex circuit 102 featureselectrodeposited copper foil with electroplated copper over the top.Thus the smaller strain limit may be applied in this example.

TABLE 1 Maximum Permissible Strain in Copper when Flex is Formed Caseinto Place Rolled annealed copper ≦16% Electrodeposited copper ≦11%

TABLE 2 Resulting Minimum Internal Bend Internal Effective Ground Radiusfor 11% Bend Substrate Plane Strain Limit Radius Thickness Thickness andNo Cover Case (inches) (inches) (inches) Layers (inches) Prior Art 0.0560.013 .002 0.069 Invention 0.056 0.005 .002 0.036

Table 2 shows the geometric parameters of the prior art. The computedminimum bend radius of 0.069 inches is greater than the previouslydescribed example of 0.040 to 0.060 inch range. Thus, theory agrees withexperiment that the flex should crack under the design parameters of theprior art.

FIGS. 5A and 5B illustrate a stackup 500 in accordance with the presentinvention. Stackup 500 at bend region 504 includes outside metal layer214 and intermediate metal layer 210; first dielectric layer 212interposed between outside metal layer 214 and intermediate metal layer210 and second dielectric layer 206 interposed between intermediatemetal layer 210 and inside metal layer 204 all stacked upon epoxy layer202. In one embodiment, inside metal layer 204 is a striplineground-plane.

In one embodiment, a portion 502 of inside metal layer 204 in bendregion 504 is etched away. The area corresponding to portion 502 ofmetal layer 204 thus removed, is then re-filled with conductive epoxy506.

FIGS. 6A and 6B illustrate a stackup 600 in accordance with the presentinvention. Stackup 600 at bend region 614 includes outside metal layer602 and second metal layer 606; dielectric layer 604 interposed betweenmetal layer 602 and metal layer 606 metal layer 606 stacked upon epoxylayer 610. In this embodiment, metal layer 606 is a microstripground-plane.

In this embodiment, a portion 612 of metal layer 606 in bend region 608is etched away. The area corresponding to portion 612 of metal layer 606thus removed, is then re-filled with conductive epoxy 608.

FIG. 9 is a flowchart illustrating a method 900 of forming a bent flexcircuit 102 in accordance with the present invention.

Referring now to FIGS. 5A, 6A and 9, in operation, a circuit 102 isformed including at least an outside metal layer 214 or 602 and aninside metal layer 204 or 606, such as copper metal layers. In step S902a portion of inside metal layer 204 or 606 is removed, such as byetching metal layer 204 or 606, from bend region 504 or 614. Afterportion 502 or 612 has been removed, in step S904 a deposition processis used to re-fill bend region 504 or 614, without using copper, torestore the electrical continuity of metal layer 204 or 606. In oneembodiment, the fill material is a conductive epoxy, like EpoxyTechnologies EE149-6.

At step S906, the fill material is subjected to B-stage curing.

Thereafter, in step S908, flex circuit 109 may be formed around mandrel104 to create the desired bend radius. The “bent” flex circuit 102 maythen be cured to cause epoxy 202 or 610 to become structural andconductive. Beneficially, epoxy 202 or 610 can be selected to duplicatethe electrical functions of the portion 502 or 612 of metal layer 204 or606 that was removed. Although, epoxy 202 or 610 was previously present,it was used to bond the copper ground plane to mandrel 104, and was nota direct part of the RF transmission structure.

This approach is advantageous because the copper is a much stiffermaterial than either the dielectric materials or the b-stage epoxyduring both elastic and plastic deformation. The difference is sopronounced that the mechanical characteristics of flex circuit 102 arealmost entirely determined by the copper metal layer.

Referring to FIGS. 4, 5 and 5B, when portion 502 is removed from metallayer 204, the neutral axis 402 shifts from the center of stackup 500(see FIG. 4) to somewhere between the two remaining metal layers, theintermediate metal aver 210 and the outer metal layer 214 as shown inFIG. 5A. Mechanically, flex circuit 102 bends almost as if it includedonly copper layers of the intermediate metal layer 210 and the outermetal layer 214, and first dielectric layer 212, even though prepreglayer 208 and second dielectric layer 206 are still present. Theeffective thickness of stackup 500 can be viewed as the combinedthicknesses of first dielectric layer 212 and copper layers of the outermetal layer 214 and the intermediate metal layer 210, which in oneembodiment is about 0.009 inches. As demonstrated in Table 2, when thepertinent parameters are applied to the IPC model, the resulting minimumbend radius becomes about 0.036 inches.

A series of electrical measurements were performed on representativeflex circuits, before and after the inside metal layer was removed.FIGS. 6 and 7 show the results of these measurements. Traces 602 and 702are the insertion and return loss signals, respectively, as measured ona typical flex circuit, Traces 604 and 704 are the insertion and returnloss signals, respectively, as measured on a modified flex circuit 102with the copper removed in accordance with the present invention. In theintended environment, the removed copper that forms part of thestripline ground-plane is replaced by the b-stage epoxy. However, forthe purpose of verification, the parts were tested with no epoxy added;this condition represents the worst-case condition. The measurementsdemonstrated there is significant change to the electrical performance.

Implementation of this invention allows flex circuit 102 to be formed toa tighter bend radii than would otherwise be possible, and allows theuse of a broader range of materials, such as the use of electrodepositedcopper rather than rolled annealed copper.

In an alternate embodiment, the flex circuit is a microstripconstruction. The microstrip construction may include a single layer ofdielectric with conductors laminated to either side. The conductor onone side is etched into one or more conducting traces, while the copperon the other side is a monolithic ground plane. The procedure previouslydescribed is equally applicable to the microstrip construction when theepoxy substitution approach is applied to the ground plane side of theflex circuit.

Although the present invention has been described with reference tospecific embodiments, these embodiments are illustrative only and notlimiting. Many other applications and embodiments of the presentinvention will be apparent in light of this disclosure and the followingclaims.

1. A flexible circuit comprising: at least an outside metal layer; atleast an inside metal layer; a first dielectric layer interposed betweenthe outside metal layer and the inside metal layer; a portion of theinside metal layer is removed to define an etched-out area and theetched out area breaks the electrical continuity of the inside metallayer; and a layer of conductive epoxy deposited on the inside metallayer and the etched-out area restores the electrical continuity of theinside metal layer and permits bending of the flexible circuit about theetched out area.
 2. The flexible circuit of claim 1, wherein the insidemetal layer forms contiguous metal sheet.
 3. The flexible circuit ofclaim 1, wherein the outside metal layer and the inside metal layer areelectrodeposited copper.
 4. The flexible circuit of claim 1, wherein theconductive epoxy covers completely the metal surface of the inside metallayer as well as the etched-out area.
 5. The flexible circuit of claim1, wherein the first dielectric layer is a low loss glass-reinforcedpolytetrafluoroethylene composite.
 6. The flexible circuit of claim 1,wherein the inside metal layer is a stripline ground-plane.
 7. Theflexible circuit of claim 1, further including an intermediate layerdisposed between the inside metal layer and the outside metal layer andwherein a neutral axis is disposed between the intermediate layer andthe outer metal layer.
 8. The flexible circuit of claim 7, furtherincluding a second dielectric layer, the first dielectric layer disposedbetween the between the outside metal layer and the intermediate layerand the second dielectric layer disposed between the intermediate layerand the inside metal layer.
 9. The flexible circuit of claim 8, furtherincluding a prepreg layer, the prepreg layer disposed between the seconddielectric layer and the intermediate layer.
 10. The flexible circuit ofclaim 6, wherein the outside metal layer is etched into a plurality ofconducting traces.